Understanding the Linux Kernel, 3rd Edition

The Process’s Address Space

The address space of a process consists of all linear addresses that the process is allowed to use. Each process sees a different set of linear addresses; the address used by one process bears no relation to the address used by another. As we will see later, the kernel may dynamically modify a process address space by adding or removing intervals of linear addresses.

The kernel represents intervals of linear addresses by means of resources called memory regions , which are characterized by an initial linear address, a length, and some access rights. For reasons of efficiency, both the initial address and the length of a memory region must be multiples of 4,096, so that the data identified by each memory region completely fills up the page frames allocated to it. Following are some typical situations in which a process gets new memory regions:

• When the user types a command at the console, the shell process creates a new process to execute the command. As a result, a fresh address space, and thus a set of memory regions, is assigned to the new process (see the section "Creating and Deleting a Process Address Space" later in this chapter; also, see Chapter 20).

• A running process may decide to load an entirely different program. In this case, the process ID remains unchanged, but the memory regions used before loading the program are released and a new set of memory regions is assigned to the process (see the section "The exec Functions" in Chapter 20).

• A running ...